I've been sitting on a piece of equipment for a little while and my Christmas project was to get the thing working again, and then make some modifications. It's around 20 years old and it's had a hard life. When it was first plugged in to the mains, nothing happened. After some initial checks of fuses it was time to remove the lid and have a look.
A poke around the PSU showed mains going in, but nothing coming out, There was also some liquid residue on the inside of the case.
Anybody who's played with old equipment will probably have experience of PSU problems - specifically with the capacitors. Over time they break down as they dry out and that can cause all sorts of problems, from low or wandering output voltages, to spectacular bangs and flashes followed by clouds of the most foul smelling smoke.
I pulled the PSU module and inspected the PSB underside. There were signs of corrosion on the board but a quick inspection of the rest of the equipment showed no such damage. This was unlikely to be water damage and so is a pretty good sign that one or more of the electrolytic capacitors have leaked.
The copper side corrosion was also under a bank of three electrolytic capacitors so I pulled them to have a look.
A poke around the PSU showed mains going in, but nothing coming out, There was also some liquid residue on the inside of the case.
Anybody who's played with old equipment will probably have experience of PSU problems - specifically with the capacitors. Over time they break down as they dry out and that can cause all sorts of problems, from low or wandering output voltages, to spectacular bangs and flashes followed by clouds of the most foul smelling smoke.
I pulled the PSU module and inspected the PSB underside. There were signs of corrosion on the board but a quick inspection of the rest of the equipment showed no such damage. This was unlikely to be water damage and so is a pretty good sign that one or more of the electrolytic capacitors have leaked.
The copper side corrosion was also under a bank of three electrolytic capacitors so I pulled them to have a look.
You can see the mess under C12. I've never figured out how the capacitor fluid manages to get through the PCB to attack the underside solder joints and tracks... but it often does. Corroded solder joints on the underside are a good indication that the component has leaked something.
A quick inventory showed there were six electrolytic capacitors on the board. Four of them were 1000uf at 35v. One was a small 100uf at 16v and a big 200uf beast rated at 450v.
This isn't my first time repairing vintage power supplies so I replaced all six. Only the big 450v capacitor could be classed as unusual and I do try and keep a few of these types in the spares box. I also used 105 degree low-impedance types for the 1000uf ones.
However, the other capacitors to consider replacing are the EMI suppression / filter capacitors. These capacitors are usually wired across the Live - Neutral and when they fail often go short circuit leaving a trail of stinky smoke and bits of capacitor housing and fluff all over the place.
Even if these capacitors are still intact, it really is just a matter of time before they fail. They may have already failed open-circuit. They only cost a few pence and take seconds to replace so you may as well do it now whilst you've got the equipment dismantled.
But this failure mode raises an important point.
You MUST replace mains side suppression / filter capacitors with at least the same voltage and X/Y class. The actual capacitance value is usually less important.
The voltage rating should be obvious, but the X/Y class can cause confusion, and getting this wrong can cause a future equipment failure to be fatal.
A quick inventory showed there were six electrolytic capacitors on the board. Four of them were 1000uf at 35v. One was a small 100uf at 16v and a big 200uf beast rated at 450v.
This isn't my first time repairing vintage power supplies so I replaced all six. Only the big 450v capacitor could be classed as unusual and I do try and keep a few of these types in the spares box. I also used 105 degree low-impedance types for the 1000uf ones.
However, the other capacitors to consider replacing are the EMI suppression / filter capacitors. These capacitors are usually wired across the Live - Neutral and when they fail often go short circuit leaving a trail of stinky smoke and bits of capacitor housing and fluff all over the place.
Even if these capacitors are still intact, it really is just a matter of time before they fail. They may have already failed open-circuit. They only cost a few pence and take seconds to replace so you may as well do it now whilst you've got the equipment dismantled.
But this failure mode raises an important point.
You MUST replace mains side suppression / filter capacitors with at least the same voltage and X/Y class. The actual capacitance value is usually less important.
The voltage rating should be obvious, but the X/Y class can cause confusion, and getting this wrong can cause a future equipment failure to be fatal.
The circuit above shows a typical basic mains filter / EMI suppression circuit though they can be much more complex of course. It's also common especially in older equipment to find very simple arrangements consisting of nothing more that a single C-X.
Looking at the above circuit, C-X is wired directly across the Live and Neutral, and the C-Y capacitors go from each line to earth.
If C-X was to fail by going open circuit, then there would be no real harm done (other than a loss of effectiveness of the circuit). If it failed by going short circuit a fuse or breaker somewhere back along the power line should blow or trip. Annoying but in the end, no serious harm done.
I've had equipment where the chassis fuse keeps blowing because the X capacitor has failed short-circuit.
If one of the C-Y capacitors were to fail open circuit again, again no real harm done. However, if they were to fail by going short-circuit a situation could arise where the chassis is left at mains voltage.
So typically class Y capacitors are designed to fail by going open circuit.
Of course, an obvious question is why don't they just use Y class capacitors in both applications. It would certainly save a lot of mess and acrid smoke being released.
I suspect the main reason is cost. Y class capacitors tend to be more expensive. A secondary consideration is probably that if a capacitor has failed the suppression characteristics of the circuit have been compromised.
Of course, nothings ever simple and there are sub groups for each of the two capacitor classes and it's important to match or exceed the specification of the faulty one.
Subgroup Peak Service Voltage
X1 > 2,500v and <= 4,000v
X2 <= 2,500v
X3 < 1,200v
Peak Service Voltage is not the components rated working voltage. The working voltage of these capacitors seems to start at around 275V AC so should be more than suitable in domestic mains equipment. X2 capacitors are very common in UK mains equipment.
Subgroup Rated Voltage
Y1 <= 500v
Y2 Between 150v and 300v
Y3 <= 250v
Y4 <= 150v
These capacitors tend to be supplied in rectangular plastic packages that can be soldered flush with the PCB. Once soldered in place, it would be almost impossible to push over without causing permanent and obvious damage to the package unlike regular ceramic disc capacitors.
They also tend to be self-healing and can often recover from a voltage spike, again unlike cheaper ceramic disc capacitors.
Looking at the above circuit, C-X is wired directly across the Live and Neutral, and the C-Y capacitors go from each line to earth.
If C-X was to fail by going open circuit, then there would be no real harm done (other than a loss of effectiveness of the circuit). If it failed by going short circuit a fuse or breaker somewhere back along the power line should blow or trip. Annoying but in the end, no serious harm done.
I've had equipment where the chassis fuse keeps blowing because the X capacitor has failed short-circuit.
If one of the C-Y capacitors were to fail open circuit again, again no real harm done. However, if they were to fail by going short-circuit a situation could arise where the chassis is left at mains voltage.
So typically class Y capacitors are designed to fail by going open circuit.
Of course, an obvious question is why don't they just use Y class capacitors in both applications. It would certainly save a lot of mess and acrid smoke being released.
I suspect the main reason is cost. Y class capacitors tend to be more expensive. A secondary consideration is probably that if a capacitor has failed the suppression characteristics of the circuit have been compromised.
Of course, nothings ever simple and there are sub groups for each of the two capacitor classes and it's important to match or exceed the specification of the faulty one.
Subgroup Peak Service Voltage
X1 > 2,500v and <= 4,000v
X2 <= 2,500v
X3 < 1,200v
Peak Service Voltage is not the components rated working voltage. The working voltage of these capacitors seems to start at around 275V AC so should be more than suitable in domestic mains equipment. X2 capacitors are very common in UK mains equipment.
Subgroup Rated Voltage
Y1 <= 500v
Y2 Between 150v and 300v
Y3 <= 250v
Y4 <= 150v
These capacitors tend to be supplied in rectangular plastic packages that can be soldered flush with the PCB. Once soldered in place, it would be almost impossible to push over without causing permanent and obvious damage to the package unlike regular ceramic disc capacitors.
They also tend to be self-healing and can often recover from a voltage spike, again unlike cheaper ceramic disc capacitors.
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